Antibodies to hepatitis A virus structural proteins have been produced in guinea pigs by inoculating either SDS disrupted purified hepatitis A virus or in rabbits by inoculating synthetic peptides. These antibodies have been used to study the structural proteins of hepatitis A virus by western blot and the antigens of the virus by in vitro neutralization and radioimmunoassays. Preliminary data indicate that VPO of HAV may not be cleaved into VP2 and VP4.